Myanmar expressed "sorrow" Monday after a bomb dropped across the border during clashes with rebel forces killed five farm workers on Chinese territory in an incident sharply condemned by Beijing.

It announced a probe into the blast, which went off in China's southwestern Yunnan province and prompted Beijing to send fighter jets to patrol the frontier, but stopped short of admitting responsibility for the incident after weeks of clashes between the Myanmar army and ethnic Kokang rebels.

"We would like to express our deep sorrow for death and injuries of Chinese nationals living in border areas," Myanmar's government said in a statement published in the state-run Global New Light of Myanmar newspaper.

It added that a joint investigation would be conducted into the incident with Beijing, which has blamed the blast on a Myanmar warplane.

The probe will also look into whether Kokang rebels were involved to "create instability along the border", the report said, adding that the military had been instructed to maintain its operations within the territory of Myanmar.

The conflict in Myanmar's remote Kokang region in northeastern Shan state sparked suddenly last month after six years of relative calm, causing tens of thousands of civilians to flee their homes, including at least 30,000 who have poured into Yunnan.

China has sent its jets to patrol the border to "warn and chase away" its neighbour's warplanes following the deadly blast in the sugarcane field during which a further eight people were injured.

On Sunday Chinese Premier Li Keqiang vowed to "resolutely" protect citizens near its Myanmar border in his once-a-year meeting with journalists.

Beijing has provided a crucial economic and political shield for the former Myanmar junta during its years of isolation.

But while Chinese investment remains paramount, Myanmar's new quasi-civilian government has sought to build greater ties with other countries including the United States since launching political reforms in 2011.